2. A flat plate is an extremely efficient reflector but has a very sharp angle of response.

3. RCS may, for practical purposes, be defined as the cross section area of a conducting sphere of such a size that it would return an echo equal in strength to that of an equivalent flat plate oriented so as to be perpendicular to the direction of the incident radiation.

4. One metre squared is the cross section of a sphere radius 0.565 metre (R2xPi - 1 metre 2)

Collision Avoidance times

Radar Interference by Sea State & Precipitation
Mariners Handbook Fourth Edition 1973

Sea' is the name given to waves generated by wind blowing locally. A radar screen becomes cluttered when echoes from waves are received. Further clutter arises from precipitation (rain, snow, and fog).

Sea States :
Moderate : 1.25 to 2.5 metres
Rough : 2.5 to 4. metres
Very rough : 4 to 6 metres

Wave Clutter:
Echoes from upwind are greater than those from seas running down wind. Wave clutter does not extend beyond five nautical miles but large echoes arise as beam grazing angles increase.

Precipitation:
Rain
Light: 4mm/hour
Moderate: 10mm/hour
Heavy: 16mm/hour

Fog is caused by the cooling of air in contact with a surface at a temperature whereby it can no longer maintain, in an invisible state, the water vapour which is present in it. Condensation of the vapour produces minute, though visible, water droplets. Rain and snow are further examples of droplets which return radar clutter.
Transmitted power, to and from the target, is attenuated by precipitation on average by -5dB (-70% of reflected power) . Visibility will be a guide to assessing power RCS lost.
Note:- Precipitation and wave clutter may or may not occur together.
The mandatory collision frequency is 'X' band. Precipitation is penetrated better on 'S' band but its target response is 1/10th 'X' band. S Band does not overcome sea clutter but may be used from beyond three nautical miles to penetrate fog, rain, snow etc. A 10m2 target will return 0.001 m2 from ten nautical miles. Doubling the distance requires target area to be16 times greater .

F. J. Wylie 'The use of Radar at Sea'

Sea Force 4 - Wave height one metre.
Sea Force 6 - Wave height Two metres.
Approximate clutter from waves:

One of the most comprehensive tests of radar reflector performance was carried out in 1995 by West Marine www.ussailing/safety/ studies/radar.reflector. The conclusions were that the poor performance of the Firdell Blipper 210/5 and 210/7 were surprising given their popularity and reputation, and although well packaged and clever device, the models tested (i.e. 210-5 and 210-7) were not large enough to have much value aboard a vessel. Similar conclusions were drawn in in the Practical Boat Owner test published in a series of articles see issue 391 July 1999. “Having so many reflectors close together produces a polar plot made up of several spikes ‘a good response' separated by an equal number of deep interference troughs in which the reflection from one corner reflector cancels out the reflection from the other.” The EchoMax range is included in the 2003 West Marine catalogue, replacing the Firdell Blipper.

The above figures illustrate the 'TriLens' 5 inch diameter reflector, for which they claim 2 to 4M2-RCS, will be hidden in moderate clutter. They also state its performance is comparable to a 12 inch corner. Presumably they mean a 12 inch octahedral RCS-2.21M2. The RCS of a single twelve inch corner is 35M2.

Rosendal's web page compares their Mini-TriLens with a Mobri reflector found 'invisible' by West Marine. The performance of other reflectors, given below, was examined by QuinetiQ in the presence of independent observers.

** Latest literature we have seen accompanying Visiball states that "its computer generated surfaces ensures a consistent performance through 360 degrees and its special filling maximizes the reflection." Our tests at QinetiQ only gave a response of 1m2 for 195 degrees. The balance of 165 degrees being virtually zero response.

For Echomax performance see website.
Echomax EM 230 weighs approximately four pounds, and comprises six identical sectors each responding with peaks to 24M2, 10M2 - 70% az., 5M2 - 100% az. When the reflector is tilted, with yaw pitch and roll, 'glint' also ensures an all round performance is maintained.
So far as we can discover we are the only manufacturer who freely publishes independent test results. We also include with each product a certified polar diagram of it performance.

ISO 8729 extends to six pages and is available from HM Stationery Office, the pertinent performance paragraphs are:

5.1.1 The maximum echoing area of the radar reflector shall be at least 10m2

5.1.2 Its azimuthal polar diagrams shall be such that its response over a total angle of 240 deg. is not less that 2.5m2. The response shall not remain below this level over any single angle of more than 10 deg.

5.1.3 These requirements shall be assessed by reference to related azimuthal polar diagrams about the reflectors vertical axis and tilted from the vertical at angles not exceeding plus or minus 3 deg.

5.2 Reflecting pattern in vertical plane The performance of the reflector up to at least plus or minus 15 deg. from the horizontal shall be such that its response at any inclination remains above 0.625m2 over a total angle of at least 240deg.

(Paragraph 5.2 is commonly accepted as flawed and was based on the performance of the 18” Octahedral.)

RORC - Royal Ocean Racing Club - ORC - Offshore Racing Council, now part of ISAF .

These operate the Offshore Special Regulations which prescribe the equipment etc., to be carried on Ocean Racing Yachts. The relevant part is regulation 4.10 which reads:

4.10 Radar reflector A Radar reflector shall be provided. If the radar reflector is octahedral it must have a minimum diagonal measurement of 456mm (18”), or if not octahedral must have a documented RCS (radar cross-section) of not less that 10m2. The minimum effective height above water is 4.0 m (13ft). Compliance with ISO 8729 is strongly recommended as a minimum standard. In addition to (but not in place of) the above, an RTE (Radar Target Enhancer) is recommended.

SOLAS Chapter V Regulation 19 para 2.1.7 (effective 01/07/2002)
Regulation 19 para.2.1.7 requires radar reflectors to be carried, where practicable, by ships under 150 GT. For UK-flagged this includes pleasure vessels.

The following notes gives further guidance on the choice of a radar reflector for small vessels and supersede Merchant Shipping Notice M.1638.

1.) Reflectors meeting the standards laid down in British Standard BS 7380:1990 (ISO standard 8729: 1987) meet IMO performance standards*. Radar reflectors which were type tested and approved to the earlier DOT Marine Radar Reflector Specification, published in 1977, also comply with the IMO standards.

2.) An important parameter of a radar reflector is it's echoing area, or equivalent radar cross-section, as this determines the amount of the radar energy which is reflected back. Reflectors to the above standards have a maximum echoing area of at least 10 m² with a minimum echoing area of at least 2.5 m² over 240° of azimuth. Orientation of the reflector must follow manufacturers recommendations if it is to be effective.

3.) Regulation 19 takes account of the fact that reflectors built to the above standards are relatively large and may not be practical for fitting to smaller vessels. The Agency considers that fitting reflectors meeting IMO standards to vessels of 15m and above length should be practicable.

4.) Owners and operators of craft vessels of less than 15m in length should fit reflectors with the greatest echoing area practical. In all cases, the reflector should be mounted as high as possible for maximum detection range, following the manufacturer's instructions.

5.) It should be noted by Master and Operators of all vessels that even the 10 m2 reflectors referred to above will be difficult to detect in sea clutter on radar displays. Masters of all vessels are reminded that this should be taken in to account when setting lookouts and determining safe speed as required by Rules 5 and 6 of the International Regulations for the Prevention of Collisions at Sea.

6.) Electronic radar target enhancers are now marketed by some manufacturers. Radar enhancers can be considered as “other means” in the Regulation. These have a larger equivalent radar cross-section for a physically smaller size than radar reflectors and produce a response on a radar display, which is stronger and more consistent, but does not increase the apparent size of the target. Some navigation buoys are being fitted with electronic radar enhancers and seafarers should be aware this improves their detection range. Mariners should note that radar enhancers currently available do not operate in the radar “S” band.

7.) Owners and operators should note that under Regulation 18 equipment meeting the requirements of Regulation 19 must be type approved. However by virtue of Regulation 1.4, the Agency allows United Kingdom vessels which are too small to fit reflectors meeting the IMO standards to fit equipment suitable for the type and size of vessel.

Paragraph 5 above confirms our view that radar reflectors with an RCS of 10m2 or less are of little or no use in real sea conditions.

The following reflectors do NOT meet ISO 8729 and therefore do not satisfy SOLAS Chapter V, RORC or ORC requirements:

- Mini-Trilens, Trilens 5.25 Inch lens, Cyclops I & II, Mobri 50mm/100mm, Blipper 210-5/210-7, Pains Wessex SC4, all Octahedral based reflectors under 18 Inches Diameter, including Davis Octahedral, VisiBall.

The EchoMax Midi meets RORC and ORC requirement of 10m2 peak but fails ISO 8729 by 2 degrees in each sector.

ECHOMAX COMMERCIAL the professionals choice

Andrew Ridley, Conservancy Operations Manager of Tees & Hartlepool Port Authority Limited first discovered Echomax at Seawork 2001. He was so impressed with their QinetiQ test results and a series of on-site tests that he insisted Echomax arrays were fitted into all the new Eason Marine buoys which he was in the process of ordering. Eason Marine, probably one of the largest suppliers of navigation buoys in the U.K, modified their top marks and cases to suit the EM230/305 arrays. Since they have been laid he has received unsolicited compliments regarding both the radar response and visibility of these units. It was confirmed that the radar response from the Eason Polyethylene 2.4m maintenance free buoy fitted with Echomax arrays was at least equal to if not better than 3m steel buoys fitted with 32" octahedral reflectors. (It is probably only the sheer mass of the metal buoys which has enabled the octahedral reflectors to be fitted for so long.)

Mr. Ridley was also receiving complaints from the Harbourmaster when another commercially available reflector fitted to a GRP pilot boat built in 2000 provided inconsistent response resulting in loss of the traces on the VTS radar. As there was insufficient mast height to fix a standard EM305, a customised 305 was fitted into a GRP roof box/mast support. Although additional height above sea level would have been beneficial it was reported that the EM305 gives a much improved response signal. An EM305 fitted to a fixed wooden finger jetty has enhanced the radar signal through all tidal states and avoids the reflection being lost when a combination of high tide and sea clutter would otherwise have caused problems to approaching vessels.

New white and orange Echomax 230's have now been fitted to all seven vessels in Tees & Hartlepool Port Authority's pilot and workboat fleet to comply with SOLAS Chapter V regulation 19

Sea Trip on Trinity House vessel Vectis to view Barrow - Black Deep 7 fitted with ECHOMAX EM 305 Top marks

After just one and a half hours steaming from Harwich on the Trinity House Vessel Vectis, Black Deep 7, green channel buoy was picked up 6.84 miles ahead. The Skipper of Vectis remarked that Echomax radar response was brighter than 11m Sunk Tower beacon with top mark. Half a mile closer we picked up the response of corresponding Trinity House Red Inner Fishers buoy. The very calm sea conditions did not demonstrate the superior response given by Echomax at heel.
All parties are aware that the Decca scanner is set very low on the Vectis mast and had this been higher all objects may well have been picked up earlier.